Skeletal Mechanism for NOx Chemistry in Diesel Engines 981450
Most computational schemes and kinetic models for engine-out NOx emissions from Diesels are based on the Zeldovich or extended Zeldovich mechanism. However, at pressures typical of both the premixed and diffusion portions of the combustion process, the third-body reaction leading to the formation of N2O (O + N2 + M) becomes faster than the leading reaction in the Zeldovich mechanism (O + N2). As in gas turbines, particularly those involving lean-premixed combustor designs, NO formation in Diesels through the N2O mechanism can thus proceed more efficiently than through the traditional route. Decomposition of NO in the combustion products during the power stroke can also occur by both the reverse Zeldovich reactions and the second order step that produces N2O (2NO ® N2O + O).
Based on these observations, a skeletal mechanism consisting of seven elementary reactions is used to develop a two-zone model for NOx emissions from direct injection (DI) Diesel engines. To evaluate the chemistry in the first zone where NO forms, the stoichiometric flame temperature and corresponding equilibrium burned gas conditions are computed at start of combustion conditions. The second zone is that in which the NO formed in zone 1 decomposes and is characterized with the equilibrium composition and flame temperature at the end of combustion, based on a fuel/air dual cycle analysis.
Characteristic chemical times for NO formation in zone 1 and NO decomposition in zone 2 are formulated from the law of mass action applied separately to each zone. The ratio of the value for decomposition to that for formation is easily computed from equilibrium analyses at the stoichiometric and overall equivalence ratios. The utility of using this ratio to evaluate the influence of decomposition upon exhaust emissions is examined. A general chart is presented, in which the logarithm of this kinetic time ratio is graphed versus reciprocal end-of-combustion flame temperature. Lines of constant pressure at start of combustion (representing turbocharger boost ratio, engine compression ratio, and injection timing) and constant peak engine pressure allow approximate positioning for any engine on the chart as a function of load once preliminary design is completed. In general, the chart suggests that decomposition of NO becomes increasingly significant as engine load (end-of-combustion flame temperature) is increased. Finally, the kinetic time ratios are computed for operating conditions typical of one light-duty and three heavy-duty turbocharged engines and found entirely consistent with results deduced from the general chart.